A patient presents with diarrhea and dehydration. A fecal smear stained by Gram's method revealed Gram-negative, comma-shaped bacteria. What is the causative agent?

Correct Answer: A

Rationale: The correct answer is A: Vibrio cholerae. Vibrio cholerae is the causative agent of cholera, which presents with severe watery diarrhea leading to dehydration. The characteristic comma-shaped (curved rod) appearance of Gram-negative bacteria is typical of Vibrio cholerae. This bacterium produces the cholera toxin, leading to the excessive secretion of fluid into the intestines. Salmonella typhi causes typhoid fever with symptoms like sustained fever, not watery diarrhea. Shigella dysenteriae causes bloody diarrhea and abdominal cramps. Escherichia coli can cause various types of infections, but typically not severe watery diarrhea like cholera.

Correct Answer: D

Rationale: Step 1: Micellar growth refers to the apical prolongation of hyphae in fungi. Step 2: Fungi have a unique growth structure with hyphae extending at the tips. Step 3: This growth pattern allows fungi to expand and access nutrients. Step 4: Bacterial cells (A) do not exhibit micellar growth. Step 5: Parent and daughter cells in division have similar sizes, not different (B). Step 6: Viruses and rickettsia (C) do not display micellar growth.

Correct Answer: D

Rationale: The correct answer is D, "All of the above," because all three bacteria listed (Vibrio cholerae, Clostridium difficile, Escherichia coli) produce toxins that can cause severe gastrointestinal illness. Vibrio cholerae produces cholera toxin leading to severe diarrhea. Clostridium difficile produces toxins causing antibiotic-associated diarrhea. Escherichia coli can produce toxins such as Shiga toxin causing severe gastrointestinal symptoms. Therefore, all the options listed are correct in producing toxins that can result in severe gastrointestinal illness.

Correct Answer: A

Rationale: The presence of flagella in a bacterial cell represents the H-antigen. Flagella are protein structures on the cell surface that play a role in bacterial motility. The H-antigen refers to the flagellar antigen, which is involved in immune responses and bacterial identification. Option A is correct because flagella are specifically associated with the H-antigen. Summarily, choices B and C are incorrect because the O-antigen and K-antigen are associated with lipopolysaccharide and capsule antigens, respectively, not flagella. Option D is incorrect as well because the correct association is with the H-antigen.

Correct Answer: C

Rationale: The correct answer is C: Obligate aerobes. These bacteria require oxygen for their survival and growth. They utilize oxygen in their metabolic processes and cannot survive in the absence of oxygen. Choice A, obligate anaerobes, cannot survive in the presence of oxygen. Choice B, facultative anaerobes, can switch between aerobic and anaerobic metabolism based on oxygen availability. Choice D, aerotolerant anaerobes, can tolerate the presence of oxygen but do not require it for their growth. Therefore, the most appropriate term for bacteria that require oxygen for survival is obligate aerobes.

Correct Answer: D

Rationale: The correct answer is D: peroxisomes. Peroxisomes contain oxidative enzymes, such as catalase and peroxidase, that are capable of oxidizing toxic substances. These enzymes help detoxify harmful molecules by breaking them down into harmless products. Lysosomes (choices B and C) contain hydrolytic enzymes for breaking down cellular waste, not oxidative enzymes. Therefore, the correct choice is peroxisomes due to their specific function in detoxification through oxidation reactions.

Correct Answer: A

Rationale: The correct answer is A because it lists the genera that belong to the family Reoviridae accurately. Orthoreovirus and Rotavirus are well-known genera within the family Reoviridae. Orbivirus is another genus belonging to the same family. Choice B is incorrect because Orthoretrovirus and Retrovirus are not part of the Reoviridae family. Orbfiloviridae is also not a valid genus within this family. Choice C is incorrect because Orthoretrovirus and Retrovirus are not part of the Reoviridae family. Orbivirus is the correct genus but the other two are inaccurate. Choice D is incorrect because Orthoretrovirus and Retrovirus are not part of the Reoviridae family. Orbarenaviridaefiloviridae is not a valid genus within Reoviridae.

Correct Answer: A

Rationale: The correct answer is A (20 min) because Escherichia coli typically has a generation time of around 20 minutes, meaning the time it takes for a bacterial population to double. This rapid reproduction rate is a key characteristic of E. coli. Option B (60 min) is incorrect as it is longer than the usual generation time. Option C (20 hours) is too long for E. coli's generation time. Option D (24 hours) is also incorrect as it is much longer than the typical generation time of E. coli.

Correct Answer: A

Rationale: The correct answer is A: Serological. Serological testing involves analyzing blood samples for specific antibodies produced in response to the pertussis infection. This method can detect past infections by identifying the presence of antibodies against the pertussis bacteria in the blood. In this case, since the doctor suspects pertussis infection in both children, serological testing can confirm if they have been exposed to the bacteria. Summary: B: Biological testing does not specifically target pertussis antibodies in the blood. C: Bacteriological testing involves culturing bacteria from samples, which may not be as sensitive for retrospective diagnosis. D: Molecular biological testing focuses on genetic material of the bacteria, which may not provide direct evidence of past infection.